A diester phosphate of the formula (I) ##STR3## (hereinafter referred to as EPC) shows superior crystallinity when converted to a salt, which is typically exemplified by a potassium salt.
Crystals of EPC monopotassium salt (hereinafter referred to as EPC-K) show less moisture absorption and are stabler than the crystals of EPC dipotassium salt (hereinafter referred to as EPC-D) and a mixture of the both salts (hereinafter referred to as EPC-KM). In addition, the surface activity of EPC-K crystals is weaker than that of EPC-D crystals or EPC-KM crystals, thus causing less mucosal irritations. Moreover, EPC-K crystals show stronger antioxidative activity, which forms the basis for their pharmacological activity, than EPC-D crystals and EPC-KM crystals.
Therefore, crystals having the highest possible EPC-K crystal content are desirable from the aspects of efficacy, stability and safety.
However, there has been conventionally made no attempt to obtain high purity EPC-K crystals, and as far as the present inventor knows, there exists no proposition to selectively produce EPC-K crystals.
It is therefore an object of the present invention to provide a method for producing high purity EPC-K crystals, whereby to provide crystals superior in efficacy, safety and stability.